The heart relies on an organized sequence of electrical impulses to beat effectively. Deviations from this normal sequence is known as arrhythmia. Certain medical devices include signal processing software that analyzes electrocardiography (ECG) signals acquired from a medical patient (e.g., a victim at a scene of an emergency) to determine when a cardiac arrhythmia such as ventricular fibrillation (VF) or shockable ventricular tachycardia (VT) exists. These devices include automated external defibrillators (AEDs), ECG rhythm classifiers, and ventricular arrhythmia detectors. An AED is a defibrillator—a device that delivers controlled electrical shock to a patient—while being relatively easy to use, such as by providing verbal prompts to a provider of care to “talk” the provider through a process of evaluating a patient for, attaching the patient to, and activating, AED therapy. Certain of the medical devices just discussed are also capable of recognizing different cardiac waveforms such normal sinus rhythm, aystole, VT and VF.
Many AEDs implement algorithms to recognize the VT and VF waveforms by performing ECG analyses at specific times during a rescue event of a patient using defibrillation and cardio-pulmonary resuscitation (CPR). The first ECG analysis is usually initiated within a few seconds after the defibrillation electrodes are attached to the patient. Typically, if the ECG analysis detects a shockable rhythm, the rescuer is advised to deliver a defibrillation shock.
Following the defibrillator shock delivery or when any of the analyses described above detects a non-shockable rhythm, treatment protocols recommended by the American Heart Association and European Resuscitation Council require performing CPR on the victim for a period of two minutes. The CPR includes rescue breathing and chest compressions. Following this period of CPR, the AED reinitiates ECG analysis as described above. The sequence of one ECG analysis/defibrillation shock followed by 2 minutes of CPR continues in a repetitive fashion for as long as the AED's power is turned on and the patient is connected to the AED device. Typically, the AED provides audio prompts to inform the rescuer when analyses are about to begin, what the analysis results were, and when to start and stop the delivery of CPR.
Many studies have reported that the discontinuation of precordial compression can significantly reduce the recovery rate of spontaneous circulation and 24-hour survival rate for victims. Thus, it is useful to recognize abnormal heart rhythms during chest compressions. There is recent clinical evidence showing that performing chest compressions before defibrillating the patient under some circumstances can be beneficial. Specifically, it is clinically beneficial to treat a patient with chest compressions before defibrillation if the response times of the medical emergency system result in a delay of more than four minutes, such that the patient is in cardiac arrest for more than four minutes. Chest compression artifact rejection can employ spectral analysis of the ECG, defibrillation success prediction, and therapeutic decision-making typically specify a set of parameters in the ECG frequency spectrum to be detected. For example, U.S. Pat. No. 5,683,424 compares a centroid or a median frequency or a peak power frequency from a calculated frequency spectrum of the ECG to thresholds to determine if a defibrillating shock is necessary.